Stormwater wetlands and ecosystem services

Stormwater Wetlands and Ecosystem Services
While they improve water quality and mitigate flooding, stormwater wetlands provide other ecosystem services that have economic, social, and ecological value. Wetland design and implementation can enhance these services and increase a stormwater wet­land’s value to the nearby community.
The use of stormwater wetlands through­out North Carolina has grown in the past decade. The principal drivers for the increased use are documented improve­ments in water quality and the state’s crediting of the nitrogen removal benefits that stormwater wetlands provide. In addi­tion to water quality improvement and flood mitigation, constructed stormwater wetlands provide other benefits or ecosys­tem services. In this design bulletin, we discuss the concept of ecosystem services and review how stormwater wetlands provide many of these services. We also describe how wetland design can enhance the provision of ecosystem services.
WHAT ARE ECOSYSTEM SERVICES?
The term ecosystem services refers to any of the benefits that ecosystems—both natural and seminatural—provide to people (MEA, 2005). These services include food and raw material provision, air and water purification, biodiversity maintenance, and aesthetic and other cultural benefits. Ecosystem services are products of the structure (for example, plant and animal community composi­tion) and processes (such as nutrient cycling and decomposition) that charac­terize an ecosystem. These services can be ascribed economic, social, and eco­logical values. Ideally, the inherent value of these services will guide management and policy decisions regarding the use and preservation of ecosystems (Figure 1). The concept of ecosystem services was introduced in the early 1970s. Since then, investigations into how our health depends on properly functioning eco­systems have surged, as have attempts to assign monetary values to ecosystem services. In one of the most widely cited ecosystem service valuation studies, Costanza et al. (1997) estimated the value of the services provided by Earth’s ecosystems to be at least $33 trillion per year. (For comparison, the global gross national product when Costanza et al. conducted their study was $18 trillion.) Of the types of ecosystems included in the study, the services provided by wetlands were among the most valuable, with a global average value of $6,000 per acre. A brief summary of the services provided by natural and created wetland ecosystems is provided in Table 1, along with the economic value of these services as estimated by Costanza et al. (1997), where applicable. 2 Urban Waterways
Figure 1. Relationship
between ecosystem
services, structure, and
processes, and their
value in decision making
(Adapted from de Groot,
2002).
Table 1. Ecosystem services provided by natural and created wetlands1
Service Examples of Goods and Services Derived Estimated value
(1994 US $/ac-1 yr-1)a
REGULATION SERVICES
Water quality
Erosion control and
sediment retention
Sediment filtration and storage capabilities that prevent downstream migration of
sediment and improve downstream water quality.
NA
Waste treatment
Reduction of excess nutrient, organic, and metal loadings reduced through microbial
degradation and/or sorption to improve water quality. Reduction of runoff tempera-ture
via shading and water’s heat capacity.
1,690
Nutrient cycling Reduction of nitrogen and phosphorus concentrations through denitrification and
biological uptake.
NA
Hydrologic regulation Moderation of the rate, volume, and frequency of surface runoff to provide flood and
storm surge protection.
1,860
Climate regulation
Greenhouse gas
regulation
Maintenance of air quality and CO2/CH4 balance (through C sequestration); regulation
of gases also influences climate effects.
54
Microclimate
regulation
Maintenance of a favorable climate (such as temperature, precipitation) for human
habitation, health, and cultivation.
NA
Soil formation Building of land surface through the accumulation of organic material in wetlands. NA
HABITAT SERVICES
Refugia
Maintenance of biological and genetic diversity through provision of suitable habitat
for resident or migratory plant and animal species. Includes the maintenance of
populations of commercially harvested species and biological pest control services.
This diversity forms the basis of many other ecosystem services.
123
PRODUCTION SERVICES
Food production Production of fish, game, fruits for small-scale hunting/gathering or aquaculture. 104
Raw materials Production of trees, peat, and other biomass appropriate for lumber, fuel, or fodder. 43
INFORMATION SERVICES
Recreation Provision of opportunities for hunting, bird-watching, hiking, or other recreational uses. 232
Cultural
Provision of opportunities for noncommercial uses, including the use of wetlands
for school excursions/education and for scientific research. Aesthetic, artistic, and
spiritual values are also included.
357
1Adapted from Costanza et al., 1997, and de Groot, 2006)
2Value estimates for each service taken from Costanza et al. (1997). A listing of NA for individual services indicates that a formal valuation of this service had not yet
been conducted.
Stormwater Wetlands and Ecosystem Services 3
ECOSYSTEM SERVICES AND STORMWATER
MANAGEMENT
As land is developed for residential and commercial
use, its ability to provide ecosystem services diminishes.
This is particularly evident in urban areas, which are
characterized by reduced flood and climate regulation
ability, poor air and water quality, and a loss of native
biodiversity. Ecologically engineered stormwater best
management practices (BMPs), however, can help
to restore the landscape’s ability to provide some of
these services. Because many of the services provided
by these engineered systems have tangible economic
value, developers and municipalities alike can benefit
by selecting stormwater practices based on the suite of
ecosystem services they provide. Additionally, space
limitations in urbanizing areas magnify the need to
design stormwater BMPs that provide flows of multiple
services – such as carbon sequestration, biodiversity,
and recreation and education opportunities – in addition
to runoff quantity and quality management.
STORMWATER WETLANDS AND ECOSYSTEM
SERVICES
Among stormwater BMPs, stormwater wetlands have
the potential to provide a great quantity and quality of
ecosystem services. Indeed, naturally-occurring and
created wetland ecosystems made up the most valu-able
terrestrial ecosystem service providers included in
Costanza et al.’s (1997) economic review. In the follow-ing
sections, we describe how wetlands, and specifically
constructed stormwater wetlands, have the potential to
provide the regulation, habitat, production, and informa-tion
services summarized in Table 1.
REGULATION SERVICES
Water treatment
Water treatment services comprise perhaps the most
widely recognized service provided by stormwater
wetlands, and much of stormwater wetland design is
geared to drive water quality benefits. As noted in Table
1, water-quality-related services include waste treat-ment,
nutrient cycling, and erosion control via sediment
and stormwater retention. Although naturally-occurring
wetlands have provided water treatment services since
the beginning of civilization, the use of constructed
wetlands as low-cost alternatives to fossil-fuel-driven
treatment technologies was not adopted in the United
States until the 1970s for wastewater treatment and until
the 1980s for stormwater treatment (Cappiella et al.,
2008). In some NC communities, constructed storm-water
wetlands have become one of the most, if not the
most, common structural stormwater control practice.
Wetlands remove and transform pollutants through a
combination of physical, chemical, and biological pro-cesses.
These complementary processes, which include
sedimentation, filtration, adsorption, chemical precipita-tion,
microbial transformation, and biological uptake,
are summarized by Hunt and Doll (2000) in Designing
Stormwater Wetlands for Small Watersheds (AG-588-2),
a publication in the Urban Waterways series published
by NC Cooperative Extension.
Nitrogen and other organic constituents are removed
from runoff primarily through the work of bacteria and
microfilms living in association with wetland plants
and sediments. The unique juxtaposition of aerobic and
anaerobic environments within wetland soils, combined
with an abundant supply of organic material, creates an
ideal environment for microbial denitrification. High
removal rates for nitrate (80 percent) and total nitro-gen
(60 percent) have been observed from stormwater
wetland systems in North Carolina (Hathaway and Hunt,
2010). Substantial phosphorus removal and total sus-pended
sediment (TSS) removal by stormwater wetlands
have also been documented. Recently, researchers inves-tigated
runoff temperature reductions by stormwater
wetlands, a service particularly beneficial in the state’s
trout-sensitive watersheds (Jones and Hunt, 2010).
Ranges of reported pollutant concentration removal rates
are displayed in Table 2 to demonstrate a stormwater
wetland’s capacity to treat stormwater for a variety of
pollutants. A wetland’s ability to mitigate nutrient loads
(nitrogen plus phosphorus) is the principal reason for the
use of constructed wetlands in North Carolina.
Table 2. Range of reported removal rates for stormwater wet-lands
with emergent vegetation (from Cappiella et al., 2008)
Pollutant 25th
percentile Median 75th
percentile
Total suspended solids 45 70 85
Total phosphorus 15 50 75
Soluble phosphorus 5 25 55
Total nitrogen 0 25 55
Organic carbon 0 20 45
Total zinc 30 40 70
Total copper 20 50 65
Bacteria 40 60 85
Hydrocarbons 50 75 90
Trash/debris 75 90 95
4 Urban Waterways
The water quality benefits provided by a wetland are
influenced by many environmental factors; however, the
wetland’s design also plays a major role in the system’s
pollutant removal capacity. Cappiella et al. (2008) stress
the importance of the following design parameters in
determining water quality benefits: the wetland’s size
relative to the target water quality volume, the surface
area to volume ratio, the length of the internal flowpath,
and the inclusion of a forebay. These parameters can be
controlled by the designer to some extent and should be
considered in the design of stormwater wetlands.
Hydrologic regulation
Due to the history of hurricane activity in North Caro-lina,
residents here are familiar with the need for hydro-logic
regulation. Hydrologic regulation services include
the regulation of the peak rate, volume, and frequency
of surface runoff from the landscape. Researchers have
identified the substantial flood control services provided
by naturally-occurring wetlands within urban areas. For
example, the U.S. Army Corps of Engineers opted to
purchase floodplain wetlands along the Charles River to
protect the city of Boston from flooding after determin-ing
that flood damages could increase by $17 million
per year if the wetlands in the river basin were drained
and disconnected from the river (Mitsch and Gosse-link,
2007). Although smaller in scale than a corridor
of floodplain wetlands, individual stormwater wetlands
also serve to regulate the flashy hydrology of urban
areas to some extent. This is typically accomplished
through peak rate control; stormwater wetlands are
usually designed to reduce peak runoff flow rates by
temporarily storing a design runoff volume (typically
from that of a 1-inch rainfall event) and slowly releasing
it over a 48-hour period (see Hunt and Doll, AG-588-2,
for more design details). Research shows that storm-water
wetlands effectively reduce peak flow rates. For
example, Line et al. (2008) reported median peak flow
reductions of 99 and 97 percent for two stormwater
wetlands in North Carolina.
Although the peak runoff rate can be controlled
solely by providing an adequate storage volume and
properly sized outlet orifice, runoff volume and fre-quency
control also rely upon evapotranspiration (ET)
and infiltration between runoff events. While ET can
represent a major outflow pathway in stormwater
wetlands, infiltration losses are often small due to high
water tables or underlying soils that are compacted
during construction to prevent the wetland from drying
out. Consequently, surface flow wetlands are gener-ally
not considered as part of the low impact develop-ment
(LID) tool palette, which emphasizes the use of
infiltration-based stormwater practices. Still, depending
on site and climatic conditions, stormwater wetlands can
appreciably reduce the volume of runoff leaving a site.
For example, a stormwater wetland in North Carolina’s
sandy coastal area reduced runoff volumes by 54 per-cent
over a 10-month monitoring period (Lenhart and
Hunt, 2011).
Climate regulation
Microclimate. Wetlands can play an important role in
climate regulation at a local scale and may also con-tribute
to climate regulation on a global scale. Climate
regulation at the local scale is of particular interest in
urban areas, where urban heat island effects may raise
the temperature by as much as 5°F (3oC). Although this
service has not been adequately quantified, the potential
cooling effects of stormwater wetlands and other green
infrastructure have been acknowledged (Bolund and
Hunhammar, 2007). The primary mechanism through
which stormwater wetlands may regulate the urban
microclimate is ET, which occurs in both the open-water
and vegetated areas of stormwater wetlands. This pro-cess
consumes a great deal of heat energy, thus helping
to regulate temperatures during the summer.
Global climate and carbon sequestration. Wetlands
are also widely recognized for their role in regulating
carbon dioxide (CO2) and methane (CH4), two green-house
gases implicated as main drivers of global climate
change. Wetland vegetation removes CO2 from the
atmosphere and stores it in above- and belowground tis-sues.
When this vegetation dies, the saturated conditions
typical of wetland soils create an anaerobic environment
in which organic matter decomposition proceeds at a
relatively slow rate, thus promoting a buildup of carbon
in the soil. Through the ongoing processes of carbon
accumulation and subsequent burial, naturally-occurring
wetlands hold massive soil carbon stores, representing
the largest component of the earth’s terrestrial biological
carbon pool, although they occupy less than 8 percent of
the earth’s surface (Mitsch and Gosselink, 2007). These
functions could be heavily valued in the future with the
coming of a carbon market and could provide another
means for developers, landowners, and others in North
Carolina to gain economic value from wetlands.
Saturated soil conditions also promote the genera-tion
of CH4, a potent greenhouse gas. CH4 is produced
when anaerobic bacteria degrade organic matter, par-ticularly
after supplies of more energetically favorable
electron acceptors, such as nitrate, manganese, iron, and
sulfate, have been exhausted. Although methanogenic
bacteria decompose organic matter slowly, significant
Stormwater Wetlands and Ecosystem Services 5
quantities of methane can be evolved; when rice pad-dies
are included, wetlands are estimated to account for
about 30 percent of global methane emissions (Mitsch
and Gosselink, 2007).
The balance between carbon sequestration and
methane production by a wetland is difficult to quantify,
and few researchers have attempted to do so. This is
particularly true of constructed stormwater wetlands, for
which carbon sequestration potential is just beginning
to be considered. Despite the lack of quantitative data,
stormwater wetlands have the potential to act as net
carbon sinks. The U.S. Geological Survey (USGS) has
documented significant quantities of carbon capture by
wetlands constructed along the Sacramento-San Joaquin
River Delta in a pilot “carbon farming” project (USGS,
2009). Though not specific to stormwater wetlands, car-bon
sequestration rates by created and restored wetland
systems have ranged from 2.7 to 4.5 tons acre-1 year-1
(Anderson and Mitsch, 2006; Euliss et al., 2006). Other
authors have noted that the carbon accumulation capac-ity
of constructed wetlands can be high, particularly
as the vegetation is establishing (Mitsch and Gosse-link,
2007). Nutrient and sediment loads delivered to
stormwater wetlands in urban runoff may also serve to
promote carbon sequestration. Nutrients promote the
growth of a productive, carbon-capturing plant com-munity
while potentially limiting methane production
by encouraging the growth of denitrifying bacteria over
methanogenic bacteria (Stadmark and Leonardson,
2005). Sediment deposition accelerates the burial of
carbon sequestered in wetland soils while presenting
the opportunity to capture carbon present in sediments
washed from the landscape (McCarty et al., 2008).
Air quality regulation
Air quality is a concern in urban and urbanizing areas,
especially where transportation and other activities
contribute to air pollution. The effect of green infra-structure,
particularly trees, on air quality in urban areas
is receiving increased attention. The main process by
which vegetation improves air quality is through physi-cal
filtering pollutants from the air, though local cli-matic
changes caused by vegetation can also impact air
quality. For example, model simulations by Taha (1997)
indicated that if tree cover in the Los Angeles area were
increased by 2 percent, the cooling effect would slow
photochemical reactions and ozone production such
that ambient air quality standards for ozone during peak
smog conditions would be exceeded 14 percent less
frequently. The air quality benefits of wetlands have
not been quantified; however, a survey of urban land
uses indicated that urban wetlands have the potential to
provide this service (Bolund and Hunhammar, 2007).
The potential for stormwater wetlands to improve air
quality by filtering particulates will depend on the types
of vegetation in the wetland and the ratio of vegetated
area to open water.
HABITAT SERVICES
Wetlands provide habitat for a wide variety of plant and
animal species, including fish, birds, amphibians, and
aquatic invertebrates. Nearly all freshwater fish depend
on wetlands for some part of their life cycle, often
laying their eggs in a wetland’s slower moving waters
during spring flooding cycles (Mitsch and Gosselink,
2007). Coastal wetlands provide valuable nursery
habitat for many saltwater species, a number of which
are commercially harvested (MEA, 2005). Wetlands
are a major provider of habitat for birds; nearly a third
of North America’s total resident bird population relies
on wetlands for some part of its life cycle (Kadlec and
Knight, 1996).
Researchers have reported that stormwater wetlands
provide habitat services when designed properly and
support diverse bird (Duffield, 1986), aquatic macro-invertebrate,
and vegetative communities (Jenkins and
Greenway, 2007) (Figure 2). However, because storm-water
wetlands serve to accumulate contaminants from
the urban landscape, some have questioned the value of
the wildlife habitat these ecosystems provide. Sparling
et al. (2004) investigated the effects of contaminant
exposure on red-winged blackbirds nesting in storm-water
wetlands near Washington, DC. They found that
the hatching success of stormwater wetland blackbird
populations compared favorably to national averages,
although zinc concentrations were elevated in the tis-sues
of birds inhabiting wetlands in industrial areas.
The authors concluded that the benefits of the habitat
provided by stormwater wetlands likely outweighed the
negative impacts of contaminant accumulation in wild-life
and that the habitat provided by stormwater wet-lands
may be especially valuable in urban areas where
such habitat is scarce. However, the long-term effects
of stormwater contaminant exposure on other wetland
biota have yet to be explored.
Wetland habitat provision is crucial as the biological
and genetic diversity maintained within a wetland forms
the basis for most of its other ecosystem services (de
Groot, 2002). For instance, a diverse plant community
may contribute to improved water treatment services
(Engelhardt and Ritchie, 2001; Line et al., 2008) and
increase the stormwater wetland’s resilience to envi-ronmental
stressors, such as pollutant pulses, extreme
climatic events, or disease (Hansson et al., 2005). The
fish and macroinvertebrate populations supported by a
6 Urban Waterways
stormwater wetland also provide a form of biological
pest control to manage mosquito populations. Storm-water
wetlands across North Carolina have been found
to support populations of mosquitofish (Gambusia
affinis), as well as aquatic macroinvertebrates such as
water boatman, backswimmers, and dragonfly larvae,
all of which prey voraciously on mosquito larvae (Hunt,
Apperson, and Lord, 2005, AG-588-4). The extent to
which each stormwater wetland provides habitat for ter-restrial
and aquatic organisms depends largely upon its
location within the urban landscape and its connectivity
with other natural ecosystems.
PRODUCTION SERVICES
Many of the plants and animals present in wetland
ecosystems can potentially provide beneficial consump-tive
uses to people, such as food and raw materials.
Ducks, geese, and other waterfowl supported by wetland
ecosystems comprise an important part of the hunting
industry. The fishing industry also relies heavily on the
provisioning services of wetlands; in 1998, the harvest
of wetland-dependent saltwater fish and shellfish totaled
nearly $950 million (Mitsch and Gosselink, 2007).
Naturally-occurring wetlands also produce an abundance
of plant species valued for timber, such as bald cypress,
tupelo, and oaks. Stormwater wetlands generate many
of the same provisioning services, though the urban
context in which they are located and the perceptions
of local citizens may limit the practicality of harvest.
However, there are possibilities. Beavers and muskrats,
which are often considered nuisances in stormwater
wetlands, could be treated as a resource by harvesting
them for their pelts. Likewise, geese, which are notori-ous
for eating young wetland vegetation, could also be
harvested as a food source. Most stormwater wetlands
support productive herbaceous vegetative communities
that hold potential as sources of energy, fiber, and other
commodities, though this potential has not been widely
explored in the United States (Mitsch and Gosselink,
2007). Many wetland plants are also edible and include
species found abundantly in stormwater wetlands, such
as duck potato (Sagitaria latifolia), cattails (Typha spp.),
and blackberries. However, contaminant accumulation in
wetland sediments may limit the value of food produc-tion
services by stormwater wetlands (Deng et al., 2004).
Until further research is conducted, it is probably not
wise to consume plants, particularly root tissues, in direct
contact with wetland sediments. The ornamental value
of some stormwater wetland plants, such as water lilies
(Nymphaea odorata), has yet to be exploited as well.
INFORMATION SERVICES
Information services contribute to our well-being by
providing information about places for recreation, edu-cation,
and aesthetic experiences as well as opportuni-ties
for reflection, spiritual enrichment, and even artistic
inspiration (de Groot, 2006). Stormwater wetlands are
particularly well-suited to provide information services
as they are located near residential areas and schools
and are often easily accessible. Increasingly, stormwater
wetlands are being integrated into urban landscapes to
provide recreational and aesthetic amenities to the sur-rounding
community (Figure 3). For instance, walking
trails, boardwalks, and wildlife viewing areas can be
maintained around and through stormwater wetlands to
provide hiking and bird-watching opportunities. Edu-
Figure 2. Stormwater
wetlands can support a
diverse vegetative com-munity,
as well as ani-mals
such as frogs and
green herons and insects
such as dragonflies.
Stormwater Wetlands and Ecosystem Services 7
cational signs can also be placed around stormwater
wetlands to inform the public of the wetlands’ regulat-ing,
habitat, and provisioning services.
Communicating the value of stormwater wetlands
as recreational and aesthetic amenities can help improve
the overall public perception of these water treatment
systems (Adams et al., 1984). The value of recreational
and aesthetic services can also translate to economic
benefits, particularly for developers. The EPA found that
homebuyers were willing to pay up to $18,000 more
for lots adjacent to aesthetically designed stormwater
wetlands and wet ponds (USEPA, 1995).
DESIGNING STORMWATER WETLANDS FOR
ECOSYSTEM SERVICES
Ultimately, the design of any stormwater wetland
system will depend on project objectives and local site
constraints. Fortunately, many of the services described
above are not mutually exclusive, so a designer can
design a system to provide multiple ecosystem ser-vices.
Because the biological and genetic diversity of
an ecosystem underpin many of the services it gener-ates,
promoting vegetative diversity through design and
construction practices also supports ecosystem services.
The following sections describe design elements that
can enhance biodiversity, and thus ecosystem service
provision, by stormwater wetlands.
Internal wetland zones
Hunt et al. (2007, AG-588-12) describe several different
hydrologic zones that can be created within stormwater
wetlands, which include deep pools, transition, shal-low
water, temporary inundation, and upper bank areas.
Including these hydrozones in stormwater wetlands will
encourage the establishment of a diverse community of
floating and emergent macrophytes, rushes, and sedges
that are adapted to varying degrees and frequencies of
inundation. Repeated wetting and drying of the areas
designed to be temporarily inundated can help facilitate
nutrient cycling, especially with respect to phospho-rus
mineralization (Bazter and Sharitz, 2006). Wetting
and drying cycles can also affect carbon sequestration.
Altor and Mitsch (2008) reported that alternating wet
and dry cycles reduced methane emissions from a cre-ated
wetland system when compared to maintaining a
steady flow hydrology. Hydrologic pulsing effects were
especially pronounced in deeper, open-water areas from
which methane emissions are generally higher and car-bon
accumulation rates lower as compared to shallower,
vegetated regions
(Anderson and
Mitsch, 2006). Thus,
managing the hydro-period
of stormwater
wetlands by incor-porating
hydrozones
could also improve
carbon sequestra-tion
services. Hunt,
Apperson, and Lord
(2005, AG-588-4)
advise incorporating
these zones so that
multiple permanent
pools are distributed throughout the wetland. Doing so
will help promulgate wetland pest control by providing
habitat for mosquito predators (such as Gambusia spp.)
throughout the wetland. Distributing deep pool refugia
throughout the wetland also enables these predators to
more rapidly recolonize shallow water areas following
drought.
Plant selection
Wetland designers can also encourage vegetative diver-sity
and the development of other ecosystem services
through the planting scheme. Selecting flowering plants
such as pickerelweed (Pontedaria cordata) adds aes-thetic
appeal while attracting mosquito predators such
as dragonflies and damselflies, which deposit their eggs
into wetland waters. The potential provisioning services
of wetland plants can also be considered when selecting
vegetation.
Soil amendments
Although wetland soils are generally rich in organic
carbon, the subsoils in which stormwater wetlands are
typically constructed are limited in organic materi-als.
Amending wetland substrates with topsoil or other
sources of organic matter at construction will help plants
Figure 3. Incor-porating
walking
trails, picnic areas,
and educational
signs (such as
these in Charlotte,
NC) enhances the
value of the infor-mation
services
provided by storm-water
wetlands.
8 Urban Waterways
establish while also providing the organic fuel needed to
drive denitrification for runoff treatment. In addition to
providing vital carbon sources, topsoil can also enhance
vegetation development and diversity through the seed-bank
and mycorrhizal bacteria already present in the soil
(Burchell et al., 2007; Cappiella et al., 2008).
Adjustable outlet structure
One challenge to maintaining a diverse vegetative com-munity
in stormwater wetlands is the drastic water level
fluctuations these systems experience during storms.
As reviewed by Cappiella et al. (2008), multiple studies
have shown that water level fluctuations greater than 8
to 10 inches above the normal water surface elevation
lead to declines in species diversity and richness. Such
water level fluctuations can be particularly detrimental
to newly planted vegetation. Hunt et al. (2007, AG-588-
12) suggest limiting the water level fluctuation to no
more than 4 to 6 inches during the first growing season.
Initial results from plant diversity and density surveys of
stormwater wetlands throughout North Carolina indicate
that maximum water level fluctuations of less than 6
inches are more likely to support a diverse plant com-munity.
An adjustable outlet structure can help to mini-mize
water level fluctuations until wetland plants are
established. Incorporating flashboard risers allows the
ponding depth to be adjusted, as described by Hunt et
al. (2007). Their Stormwater Wetland Design Update
(AG-588-12) provides design guidance for outlet struc-tures.
Further construction guidance for these systems is
provided by Burchell et al. (2010) in Stormwater Wet-land
Construction Practices (AG-588-13).
SUMMARY
Naturally-occurring wetlands are recognized as one of
the world’s most valuable ecosystems by virtue of the
free services they provide to society, including water
treatment; flood and greenhouse gas regulation; biodi-versity
maintenance; food and raw material production;
and recreational, educational, and aesthetic experiences.
Though currently designed for runoff treatment in North
Carolina and, to some degree, flood regulation, storm-water
wetlands may provide many of the other services
provided by naturally-occurring wetlands. Because they
are generally located in urban areas, stormwater wet-lands
have the potential to provide air and microclimate
regulation services. This potential in particular merits
further investigation by researchers and others involved
in stormwater mitigation. Carbon sequestration by
stormwater wetlands is another area for future explora-tion
with potential economic benefits for developers and
NC municipalities through the carbon market.
The types of ecosystem services provided by a
stormwater wetland will partly depend on its design.
Current design guidance, particularly including vari-ous
hydrozones and maintaining maximum water level
fluctuations to less than 6 inches while vegetation is
establishing, will encourage a more diverse community
of wetland vegetation. Much of the literature points to
this diversity as a driver for providing other ecosystem
services.
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and natural capital. Nature 387: 253-260.
Stormwater Wetlands and Ecosystem Services 9
de Groot, R.S., Wilson, M.A., and Boumans, R.M. 2002.
A typology for the classification, description and
valuation of ecosystem functions, goods and services.
Ecological Economics 41: 393-408.
de Groot, R.S. 2006. Function-analysis and valuation
as a tool to assess land use conflicts in planning for
sustainable, multi-functional landscapes. Landscape
and Urban Planning 75(3-4): 175-186.
Deng, H., Z. H. Ye, and M. H. Wong. 2004. Accumula-tion
of lead, zinc, copper and cadmium by 12 wetland
plant species thriving in metal-contaminated sites in
China. Environmental Pollution 132(1): 29-40.
Duffield, J. M. 1986. Waterbird use of an urban storm-water
wetland system in Central California, USA.
Colonial Waterbirds 9(2): 227-235.
Hansson, L. A, C. Bronmark, P. Anders Nilsson, K.
Abjornsson. 2005. Conflicting demands on wetland
ecosystem services: nutrient retention, biodiversity or
both? Freshwater Biology 50: 705-714.
Hathaway, J. M. and W. F. Hunt. 2010. Evaluation of
storm-water wetlands in series in Piedmont North
Carolina. Journal of Environmental Engineering
136(1): 140-146.
Jenkins, G. A. and M. Greenway. 2007. Restoration of
a constructed stormwater wetland to improve its eco-logical
and hydrological performance. Water Science
& Technology 56(11): 109-116.
Kadlec, R. and R. Knight. 1996. Treatment Wetlands.
Boca Raton, FL: CRC Press.
Jones, M. P. and W. F. Hunt. 2010. Effect of stormwa-ter
wetlands and wet ponds on runoff temperature
in trout sensitive waters. Journal of Irrigation and
Drainage Engineering (in press, September 2010).
Lenhart, H. A. and W. F. Hunt. 2011. Evaluating four
stormwater performance metrics with a North Caro-lina
Coastal Plain stormwater wetland. Journal of
Environmental Engineering. 137(2): TBD.
Line, D. E., G. D. Jennings, M. B. Shaffer, J. Calabria,
and W. F. Hunt. 2008. Evaluating the effectiveness of
two stormwater wetlands in North Carolina. Transac-tions
of the ASABE 51(2): 521-528.
Millennium Ecosystem Assessment. 2005. Ecosystem
services and human well-being: Wetlands and water:
Synthesis. Report to the Ramsar Convention. Wash-ington,
DC: World Resources Institute. [Cited Sept.
3, 2008] Online: http://www.millenniumassessment.
org/en/aspx.
Mitsh, W. J. and J. G. Gosselink. 2007. Wetlands. Hobo-ken,
NJ: John Wiley and Sons, Inc.
Sparling D.W., J. D. Eisemann, and W. Kuenzel. 2004.
Contaminant exposure and effects in red-winged
blackbirds inhabiting stormwater retention ponds.
Environmental Management 33(5): 719-729.
Stadmark, J. and L. Leonardson. 2005. Emissions of
greenhouse gases from ponds constructed for nitrogen
removal. Ecological Engineering 25(5): 542-551.
U.S. EPA. 1995. Economic Benefits of Runoff Controls.
EPA Document No. 841-5-95-002. Washington, DC:
Office of Wetlands, Oceans and Watersheds.
U.S. Geological Survey (USGS). 2009. California Water
Science Center: Carbon Farming. Sacramento, CA:
California Water Science Center, USGS. Online:
http://ca.water.usgs.gov/Carbon_Farm/index.html.
RELATED FACT SHEETS
These fact sheets in the Urban Waterways series (AG-
588) published by the NC Cooperative Extension Ser-vice
at NC State University, Raleigh, are available on
the Stormwater Engineering website: http://www.bae.
ncsu.edu/stormwater/pubs.html
Hunt, W. F. and B.A. Doll. 2000. Designing Stormwater
Wetlands for Small Watersheds (AG-588-2).
Hunt, W. F., C. S. Apperson, and W. G. Lord. 2005.
Mosquito Control for Stormwater Facilities (AG-588-
4).
Hunt, W. F. and W. G. Lord. 2006. Stormwater Wetland
and Wet Pond Maintenance (AG-588-7).
Jones, M. P. and W. F. Hunt. 2007. Designing Urban
Stormwater BMPs for Trout Waters (AG-588-11).
Hunt, W. F., M. R. Burchell, J. D. Wright, and K. L.
Bass. 2007. Stormwater Wetland Design Update
(AG-588-12).
Burchell, M. R., W. F. Hunt, K. L. Bass, and J. D.
Wright. 2011. Stormwater Wetland Construction
Practices (AG-588-13).
Hathaway, J. M. and W.F. Hunt. 2008. Removal of
Pathogens in Stormwater (AG-588-16W).
NC STATE UNIVERSITY
Distributed in furtherance of the acts of Congress of May 8 and June 30, 1914. North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, veteran status or disability. In addition, the two Universities welcome all persons without regard to sexual orientation. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
Published by
NORTH CAROLINA COOPERATIVE EXTENSION
11-CALS-2352 AGW-588-22
1/11—BS/KEL
Prepared by
Trisha L. Moore, Graduate Teaching and Research Assistant
William F. Hunt III, Ph.D., PE, Associate Professor and Extension Specialist
Department of Biological and Agricultural Engineering

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Stormwater Wetlands and Ecosystem Services
While they improve water quality and mitigate flooding, stormwater wetlands provide other ecosystem services that have economic, social, and ecological value. Wetland design and implementation can enhance these services and increase a stormwater wet­land’s value to the nearby community.
The use of stormwater wetlands through­out North Carolina has grown in the past decade. The principal drivers for the increased use are documented improve­ments in water quality and the state’s crediting of the nitrogen removal benefits that stormwater wetlands provide. In addi­tion to water quality improvement and flood mitigation, constructed stormwater wetlands provide other benefits or ecosys­tem services. In this design bulletin, we discuss the concept of ecosystem services and review how stormwater wetlands provide many of these services. We also describe how wetland design can enhance the provision of ecosystem services.
WHAT ARE ECOSYSTEM SERVICES?
The term ecosystem services refers to any of the benefits that ecosystems—both natural and seminatural—provide to people (MEA, 2005). These services include food and raw material provision, air and water purification, biodiversity maintenance, and aesthetic and other cultural benefits. Ecosystem services are products of the structure (for example, plant and animal community composi­tion) and processes (such as nutrient cycling and decomposition) that charac­terize an ecosystem. These services can be ascribed economic, social, and eco­logical values. Ideally, the inherent value of these services will guide management and policy decisions regarding the use and preservation of ecosystems (Figure 1). The concept of ecosystem services was introduced in the early 1970s. Since then, investigations into how our health depends on properly functioning eco­systems have surged, as have attempts to assign monetary values to ecosystem services. In one of the most widely cited ecosystem service valuation studies, Costanza et al. (1997) estimated the value of the services provided by Earth’s ecosystems to be at least $33 trillion per year. (For comparison, the global gross national product when Costanza et al. conducted their study was $18 trillion.) Of the types of ecosystems included in the study, the services provided by wetlands were among the most valuable, with a global average value of $6,000 per acre. A brief summary of the services provided by natural and created wetland ecosystems is provided in Table 1, along with the economic value of these services as estimated by Costanza et al. (1997), where applicable. 2 Urban Waterways
Figure 1. Relationship
between ecosystem
services, structure, and
processes, and their
value in decision making
(Adapted from de Groot,
2002).
Table 1. Ecosystem services provided by natural and created wetlands1
Service Examples of Goods and Services Derived Estimated value
(1994 US $/ac-1 yr-1)a
REGULATION SERVICES
Water quality
Erosion control and
sediment retention
Sediment filtration and storage capabilities that prevent downstream migration of
sediment and improve downstream water quality.
NA
Waste treatment
Reduction of excess nutrient, organic, and metal loadings reduced through microbial
degradation and/or sorption to improve water quality. Reduction of runoff tempera-ture
via shading and water’s heat capacity.
1,690
Nutrient cycling Reduction of nitrogen and phosphorus concentrations through denitrification and
biological uptake.
NA
Hydrologic regulation Moderation of the rate, volume, and frequency of surface runoff to provide flood and
storm surge protection.
1,860
Climate regulation
Greenhouse gas
regulation
Maintenance of air quality and CO2/CH4 balance (through C sequestration); regulation
of gases also influences climate effects.
54
Microclimate
regulation
Maintenance of a favorable climate (such as temperature, precipitation) for human
habitation, health, and cultivation.
NA
Soil formation Building of land surface through the accumulation of organic material in wetlands. NA
HABITAT SERVICES
Refugia
Maintenance of biological and genetic diversity through provision of suitable habitat
for resident or migratory plant and animal species. Includes the maintenance of
populations of commercially harvested species and biological pest control services.
This diversity forms the basis of many other ecosystem services.
123
PRODUCTION SERVICES
Food production Production of fish, game, fruits for small-scale hunting/gathering or aquaculture. 104
Raw materials Production of trees, peat, and other biomass appropriate for lumber, fuel, or fodder. 43
INFORMATION SERVICES
Recreation Provision of opportunities for hunting, bird-watching, hiking, or other recreational uses. 232
Cultural
Provision of opportunities for noncommercial uses, including the use of wetlands
for school excursions/education and for scientific research. Aesthetic, artistic, and
spiritual values are also included.
357
1Adapted from Costanza et al., 1997, and de Groot, 2006)
2Value estimates for each service taken from Costanza et al. (1997). A listing of NA for individual services indicates that a formal valuation of this service had not yet
been conducted.
Stormwater Wetlands and Ecosystem Services 3
ECOSYSTEM SERVICES AND STORMWATER
MANAGEMENT
As land is developed for residential and commercial
use, its ability to provide ecosystem services diminishes.
This is particularly evident in urban areas, which are
characterized by reduced flood and climate regulation
ability, poor air and water quality, and a loss of native
biodiversity. Ecologically engineered stormwater best
management practices (BMPs), however, can help
to restore the landscape’s ability to provide some of
these services. Because many of the services provided
by these engineered systems have tangible economic
value, developers and municipalities alike can benefit
by selecting stormwater practices based on the suite of
ecosystem services they provide. Additionally, space
limitations in urbanizing areas magnify the need to
design stormwater BMPs that provide flows of multiple
services – such as carbon sequestration, biodiversity,
and recreation and education opportunities – in addition
to runoff quantity and quality management.
STORMWATER WETLANDS AND ECOSYSTEM
SERVICES
Among stormwater BMPs, stormwater wetlands have
the potential to provide a great quantity and quality of
ecosystem services. Indeed, naturally-occurring and
created wetland ecosystems made up the most valu-able
terrestrial ecosystem service providers included in
Costanza et al.’s (1997) economic review. In the follow-ing
sections, we describe how wetlands, and specifically
constructed stormwater wetlands, have the potential to
provide the regulation, habitat, production, and informa-tion
services summarized in Table 1.
REGULATION SERVICES
Water treatment
Water treatment services comprise perhaps the most
widely recognized service provided by stormwater
wetlands, and much of stormwater wetland design is
geared to drive water quality benefits. As noted in Table
1, water-quality-related services include waste treat-ment,
nutrient cycling, and erosion control via sediment
and stormwater retention. Although naturally-occurring
wetlands have provided water treatment services since
the beginning of civilization, the use of constructed
wetlands as low-cost alternatives to fossil-fuel-driven
treatment technologies was not adopted in the United
States until the 1970s for wastewater treatment and until
the 1980s for stormwater treatment (Cappiella et al.,
2008). In some NC communities, constructed storm-water
wetlands have become one of the most, if not the
most, common structural stormwater control practice.
Wetlands remove and transform pollutants through a
combination of physical, chemical, and biological pro-cesses.
These complementary processes, which include
sedimentation, filtration, adsorption, chemical precipita-tion,
microbial transformation, and biological uptake,
are summarized by Hunt and Doll (2000) in Designing
Stormwater Wetlands for Small Watersheds (AG-588-2),
a publication in the Urban Waterways series published
by NC Cooperative Extension.
Nitrogen and other organic constituents are removed
from runoff primarily through the work of bacteria and
microfilms living in association with wetland plants
and sediments. The unique juxtaposition of aerobic and
anaerobic environments within wetland soils, combined
with an abundant supply of organic material, creates an
ideal environment for microbial denitrification. High
removal rates for nitrate (80 percent) and total nitro-gen
(60 percent) have been observed from stormwater
wetland systems in North Carolina (Hathaway and Hunt,
2010). Substantial phosphorus removal and total sus-pended
sediment (TSS) removal by stormwater wetlands
have also been documented. Recently, researchers inves-tigated
runoff temperature reductions by stormwater
wetlands, a service particularly beneficial in the state’s
trout-sensitive watersheds (Jones and Hunt, 2010).
Ranges of reported pollutant concentration removal rates
are displayed in Table 2 to demonstrate a stormwater
wetland’s capacity to treat stormwater for a variety of
pollutants. A wetland’s ability to mitigate nutrient loads
(nitrogen plus phosphorus) is the principal reason for the
use of constructed wetlands in North Carolina.
Table 2. Range of reported removal rates for stormwater wet-lands
with emergent vegetation (from Cappiella et al., 2008)
Pollutant 25th
percentile Median 75th
percentile
Total suspended solids 45 70 85
Total phosphorus 15 50 75
Soluble phosphorus 5 25 55
Total nitrogen 0 25 55
Organic carbon 0 20 45
Total zinc 30 40 70
Total copper 20 50 65
Bacteria 40 60 85
Hydrocarbons 50 75 90
Trash/debris 75 90 95
4 Urban Waterways
The water quality benefits provided by a wetland are
influenced by many environmental factors; however, the
wetland’s design also plays a major role in the system’s
pollutant removal capacity. Cappiella et al. (2008) stress
the importance of the following design parameters in
determining water quality benefits: the wetland’s size
relative to the target water quality volume, the surface
area to volume ratio, the length of the internal flowpath,
and the inclusion of a forebay. These parameters can be
controlled by the designer to some extent and should be
considered in the design of stormwater wetlands.
Hydrologic regulation
Due to the history of hurricane activity in North Caro-lina,
residents here are familiar with the need for hydro-logic
regulation. Hydrologic regulation services include
the regulation of the peak rate, volume, and frequency
of surface runoff from the landscape. Researchers have
identified the substantial flood control services provided
by naturally-occurring wetlands within urban areas. For
example, the U.S. Army Corps of Engineers opted to
purchase floodplain wetlands along the Charles River to
protect the city of Boston from flooding after determin-ing
that flood damages could increase by $17 million
per year if the wetlands in the river basin were drained
and disconnected from the river (Mitsch and Gosse-link,
2007). Although smaller in scale than a corridor
of floodplain wetlands, individual stormwater wetlands
also serve to regulate the flashy hydrology of urban
areas to some extent. This is typically accomplished
through peak rate control; stormwater wetlands are
usually designed to reduce peak runoff flow rates by
temporarily storing a design runoff volume (typically
from that of a 1-inch rainfall event) and slowly releasing
it over a 48-hour period (see Hunt and Doll, AG-588-2,
for more design details). Research shows that storm-water
wetlands effectively reduce peak flow rates. For
example, Line et al. (2008) reported median peak flow
reductions of 99 and 97 percent for two stormwater
wetlands in North Carolina.
Although the peak runoff rate can be controlled
solely by providing an adequate storage volume and
properly sized outlet orifice, runoff volume and fre-quency
control also rely upon evapotranspiration (ET)
and infiltration between runoff events. While ET can
represent a major outflow pathway in stormwater
wetlands, infiltration losses are often small due to high
water tables or underlying soils that are compacted
during construction to prevent the wetland from drying
out. Consequently, surface flow wetlands are gener-ally
not considered as part of the low impact develop-ment
(LID) tool palette, which emphasizes the use of
infiltration-based stormwater practices. Still, depending
on site and climatic conditions, stormwater wetlands can
appreciably reduce the volume of runoff leaving a site.
For example, a stormwater wetland in North Carolina’s
sandy coastal area reduced runoff volumes by 54 per-cent
over a 10-month monitoring period (Lenhart and
Hunt, 2011).
Climate regulation
Microclimate. Wetlands can play an important role in
climate regulation at a local scale and may also con-tribute
to climate regulation on a global scale. Climate
regulation at the local scale is of particular interest in
urban areas, where urban heat island effects may raise
the temperature by as much as 5°F (3oC). Although this
service has not been adequately quantified, the potential
cooling effects of stormwater wetlands and other green
infrastructure have been acknowledged (Bolund and
Hunhammar, 2007). The primary mechanism through
which stormwater wetlands may regulate the urban
microclimate is ET, which occurs in both the open-water
and vegetated areas of stormwater wetlands. This pro-cess
consumes a great deal of heat energy, thus helping
to regulate temperatures during the summer.
Global climate and carbon sequestration. Wetlands
are also widely recognized for their role in regulating
carbon dioxide (CO2) and methane (CH4), two green-house
gases implicated as main drivers of global climate
change. Wetland vegetation removes CO2 from the
atmosphere and stores it in above- and belowground tis-sues.
When this vegetation dies, the saturated conditions
typical of wetland soils create an anaerobic environment
in which organic matter decomposition proceeds at a
relatively slow rate, thus promoting a buildup of carbon
in the soil. Through the ongoing processes of carbon
accumulation and subsequent burial, naturally-occurring
wetlands hold massive soil carbon stores, representing
the largest component of the earth’s terrestrial biological
carbon pool, although they occupy less than 8 percent of
the earth’s surface (Mitsch and Gosselink, 2007). These
functions could be heavily valued in the future with the
coming of a carbon market and could provide another
means for developers, landowners, and others in North
Carolina to gain economic value from wetlands.
Saturated soil conditions also promote the genera-tion
of CH4, a potent greenhouse gas. CH4 is produced
when anaerobic bacteria degrade organic matter, par-ticularly
after supplies of more energetically favorable
electron acceptors, such as nitrate, manganese, iron, and
sulfate, have been exhausted. Although methanogenic
bacteria decompose organic matter slowly, significant
Stormwater Wetlands and Ecosystem Services 5
quantities of methane can be evolved; when rice pad-dies
are included, wetlands are estimated to account for
about 30 percent of global methane emissions (Mitsch
and Gosselink, 2007).
The balance between carbon sequestration and
methane production by a wetland is difficult to quantify,
and few researchers have attempted to do so. This is
particularly true of constructed stormwater wetlands, for
which carbon sequestration potential is just beginning
to be considered. Despite the lack of quantitative data,
stormwater wetlands have the potential to act as net
carbon sinks. The U.S. Geological Survey (USGS) has
documented significant quantities of carbon capture by
wetlands constructed along the Sacramento-San Joaquin
River Delta in a pilot “carbon farming” project (USGS,
2009). Though not specific to stormwater wetlands, car-bon
sequestration rates by created and restored wetland
systems have ranged from 2.7 to 4.5 tons acre-1 year-1
(Anderson and Mitsch, 2006; Euliss et al., 2006). Other
authors have noted that the carbon accumulation capac-ity
of constructed wetlands can be high, particularly
as the vegetation is establishing (Mitsch and Gosse-link,
2007). Nutrient and sediment loads delivered to
stormwater wetlands in urban runoff may also serve to
promote carbon sequestration. Nutrients promote the
growth of a productive, carbon-capturing plant com-munity
while potentially limiting methane production
by encouraging the growth of denitrifying bacteria over
methanogenic bacteria (Stadmark and Leonardson,
2005). Sediment deposition accelerates the burial of
carbon sequestered in wetland soils while presenting
the opportunity to capture carbon present in sediments
washed from the landscape (McCarty et al., 2008).
Air quality regulation
Air quality is a concern in urban and urbanizing areas,
especially where transportation and other activities
contribute to air pollution. The effect of green infra-structure,
particularly trees, on air quality in urban areas
is receiving increased attention. The main process by
which vegetation improves air quality is through physi-cal
filtering pollutants from the air, though local cli-matic
changes caused by vegetation can also impact air
quality. For example, model simulations by Taha (1997)
indicated that if tree cover in the Los Angeles area were
increased by 2 percent, the cooling effect would slow
photochemical reactions and ozone production such
that ambient air quality standards for ozone during peak
smog conditions would be exceeded 14 percent less
frequently. The air quality benefits of wetlands have
not been quantified; however, a survey of urban land
uses indicated that urban wetlands have the potential to
provide this service (Bolund and Hunhammar, 2007).
The potential for stormwater wetlands to improve air
quality by filtering particulates will depend on the types
of vegetation in the wetland and the ratio of vegetated
area to open water.
HABITAT SERVICES
Wetlands provide habitat for a wide variety of plant and
animal species, including fish, birds, amphibians, and
aquatic invertebrates. Nearly all freshwater fish depend
on wetlands for some part of their life cycle, often
laying their eggs in a wetland’s slower moving waters
during spring flooding cycles (Mitsch and Gosselink,
2007). Coastal wetlands provide valuable nursery
habitat for many saltwater species, a number of which
are commercially harvested (MEA, 2005). Wetlands
are a major provider of habitat for birds; nearly a third
of North America’s total resident bird population relies
on wetlands for some part of its life cycle (Kadlec and
Knight, 1996).
Researchers have reported that stormwater wetlands
provide habitat services when designed properly and
support diverse bird (Duffield, 1986), aquatic macro-invertebrate,
and vegetative communities (Jenkins and
Greenway, 2007) (Figure 2). However, because storm-water
wetlands serve to accumulate contaminants from
the urban landscape, some have questioned the value of
the wildlife habitat these ecosystems provide. Sparling
et al. (2004) investigated the effects of contaminant
exposure on red-winged blackbirds nesting in storm-water
wetlands near Washington, DC. They found that
the hatching success of stormwater wetland blackbird
populations compared favorably to national averages,
although zinc concentrations were elevated in the tis-sues
of birds inhabiting wetlands in industrial areas.
The authors concluded that the benefits of the habitat
provided by stormwater wetlands likely outweighed the
negative impacts of contaminant accumulation in wild-life
and that the habitat provided by stormwater wet-lands
may be especially valuable in urban areas where
such habitat is scarce. However, the long-term effects
of stormwater contaminant exposure on other wetland
biota have yet to be explored.
Wetland habitat provision is crucial as the biological
and genetic diversity maintained within a wetland forms
the basis for most of its other ecosystem services (de
Groot, 2002). For instance, a diverse plant community
may contribute to improved water treatment services
(Engelhardt and Ritchie, 2001; Line et al., 2008) and
increase the stormwater wetland’s resilience to envi-ronmental
stressors, such as pollutant pulses, extreme
climatic events, or disease (Hansson et al., 2005). The
fish and macroinvertebrate populations supported by a
6 Urban Waterways
stormwater wetland also provide a form of biological
pest control to manage mosquito populations. Storm-water
wetlands across North Carolina have been found
to support populations of mosquitofish (Gambusia
affinis), as well as aquatic macroinvertebrates such as
water boatman, backswimmers, and dragonfly larvae,
all of which prey voraciously on mosquito larvae (Hunt,
Apperson, and Lord, 2005, AG-588-4). The extent to
which each stormwater wetland provides habitat for ter-restrial
and aquatic organisms depends largely upon its
location within the urban landscape and its connectivity
with other natural ecosystems.
PRODUCTION SERVICES
Many of the plants and animals present in wetland
ecosystems can potentially provide beneficial consump-tive
uses to people, such as food and raw materials.
Ducks, geese, and other waterfowl supported by wetland
ecosystems comprise an important part of the hunting
industry. The fishing industry also relies heavily on the
provisioning services of wetlands; in 1998, the harvest
of wetland-dependent saltwater fish and shellfish totaled
nearly $950 million (Mitsch and Gosselink, 2007).
Naturally-occurring wetlands also produce an abundance
of plant species valued for timber, such as bald cypress,
tupelo, and oaks. Stormwater wetlands generate many
of the same provisioning services, though the urban
context in which they are located and the perceptions
of local citizens may limit the practicality of harvest.
However, there are possibilities. Beavers and muskrats,
which are often considered nuisances in stormwater
wetlands, could be treated as a resource by harvesting
them for their pelts. Likewise, geese, which are notori-ous
for eating young wetland vegetation, could also be
harvested as a food source. Most stormwater wetlands
support productive herbaceous vegetative communities
that hold potential as sources of energy, fiber, and other
commodities, though this potential has not been widely
explored in the United States (Mitsch and Gosselink,
2007). Many wetland plants are also edible and include
species found abundantly in stormwater wetlands, such
as duck potato (Sagitaria latifolia), cattails (Typha spp.),
and blackberries. However, contaminant accumulation in
wetland sediments may limit the value of food produc-tion
services by stormwater wetlands (Deng et al., 2004).
Until further research is conducted, it is probably not
wise to consume plants, particularly root tissues, in direct
contact with wetland sediments. The ornamental value
of some stormwater wetland plants, such as water lilies
(Nymphaea odorata), has yet to be exploited as well.
INFORMATION SERVICES
Information services contribute to our well-being by
providing information about places for recreation, edu-cation,
and aesthetic experiences as well as opportuni-ties
for reflection, spiritual enrichment, and even artistic
inspiration (de Groot, 2006). Stormwater wetlands are
particularly well-suited to provide information services
as they are located near residential areas and schools
and are often easily accessible. Increasingly, stormwater
wetlands are being integrated into urban landscapes to
provide recreational and aesthetic amenities to the sur-rounding
community (Figure 3). For instance, walking
trails, boardwalks, and wildlife viewing areas can be
maintained around and through stormwater wetlands to
provide hiking and bird-watching opportunities. Edu-
Figure 2. Stormwater
wetlands can support a
diverse vegetative com-munity,
as well as ani-mals
such as frogs and
green herons and insects
such as dragonflies.
Stormwater Wetlands and Ecosystem Services 7
cational signs can also be placed around stormwater
wetlands to inform the public of the wetlands’ regulat-ing,
habitat, and provisioning services.
Communicating the value of stormwater wetlands
as recreational and aesthetic amenities can help improve
the overall public perception of these water treatment
systems (Adams et al., 1984). The value of recreational
and aesthetic services can also translate to economic
benefits, particularly for developers. The EPA found that
homebuyers were willing to pay up to $18,000 more
for lots adjacent to aesthetically designed stormwater
wetlands and wet ponds (USEPA, 1995).
DESIGNING STORMWATER WETLANDS FOR
ECOSYSTEM SERVICES
Ultimately, the design of any stormwater wetland
system will depend on project objectives and local site
constraints. Fortunately, many of the services described
above are not mutually exclusive, so a designer can
design a system to provide multiple ecosystem ser-vices.
Because the biological and genetic diversity of
an ecosystem underpin many of the services it gener-ates,
promoting vegetative diversity through design and
construction practices also supports ecosystem services.
The following sections describe design elements that
can enhance biodiversity, and thus ecosystem service
provision, by stormwater wetlands.
Internal wetland zones
Hunt et al. (2007, AG-588-12) describe several different
hydrologic zones that can be created within stormwater
wetlands, which include deep pools, transition, shal-low
water, temporary inundation, and upper bank areas.
Including these hydrozones in stormwater wetlands will
encourage the establishment of a diverse community of
floating and emergent macrophytes, rushes, and sedges
that are adapted to varying degrees and frequencies of
inundation. Repeated wetting and drying of the areas
designed to be temporarily inundated can help facilitate
nutrient cycling, especially with respect to phospho-rus
mineralization (Bazter and Sharitz, 2006). Wetting
and drying cycles can also affect carbon sequestration.
Altor and Mitsch (2008) reported that alternating wet
and dry cycles reduced methane emissions from a cre-ated
wetland system when compared to maintaining a
steady flow hydrology. Hydrologic pulsing effects were
especially pronounced in deeper, open-water areas from
which methane emissions are generally higher and car-bon
accumulation rates lower as compared to shallower,
vegetated regions
(Anderson and
Mitsch, 2006). Thus,
managing the hydro-period
of stormwater
wetlands by incor-porating
hydrozones
could also improve
carbon sequestra-tion
services. Hunt,
Apperson, and Lord
(2005, AG-588-4)
advise incorporating
these zones so that
multiple permanent
pools are distributed throughout the wetland. Doing so
will help promulgate wetland pest control by providing
habitat for mosquito predators (such as Gambusia spp.)
throughout the wetland. Distributing deep pool refugia
throughout the wetland also enables these predators to
more rapidly recolonize shallow water areas following
drought.
Plant selection
Wetland designers can also encourage vegetative diver-sity
and the development of other ecosystem services
through the planting scheme. Selecting flowering plants
such as pickerelweed (Pontedaria cordata) adds aes-thetic
appeal while attracting mosquito predators such
as dragonflies and damselflies, which deposit their eggs
into wetland waters. The potential provisioning services
of wetland plants can also be considered when selecting
vegetation.
Soil amendments
Although wetland soils are generally rich in organic
carbon, the subsoils in which stormwater wetlands are
typically constructed are limited in organic materi-als.
Amending wetland substrates with topsoil or other
sources of organic matter at construction will help plants
Figure 3. Incor-porating
walking
trails, picnic areas,
and educational
signs (such as
these in Charlotte,
NC) enhances the
value of the infor-mation
services
provided by storm-water
wetlands.
8 Urban Waterways
establish while also providing the organic fuel needed to
drive denitrification for runoff treatment. In addition to
providing vital carbon sources, topsoil can also enhance
vegetation development and diversity through the seed-bank
and mycorrhizal bacteria already present in the soil
(Burchell et al., 2007; Cappiella et al., 2008).
Adjustable outlet structure
One challenge to maintaining a diverse vegetative com-munity
in stormwater wetlands is the drastic water level
fluctuations these systems experience during storms.
As reviewed by Cappiella et al. (2008), multiple studies
have shown that water level fluctuations greater than 8
to 10 inches above the normal water surface elevation
lead to declines in species diversity and richness. Such
water level fluctuations can be particularly detrimental
to newly planted vegetation. Hunt et al. (2007, AG-588-
12) suggest limiting the water level fluctuation to no
more than 4 to 6 inches during the first growing season.
Initial results from plant diversity and density surveys of
stormwater wetlands throughout North Carolina indicate
that maximum water level fluctuations of less than 6
inches are more likely to support a diverse plant com-munity.
An adjustable outlet structure can help to mini-mize
water level fluctuations until wetland plants are
established. Incorporating flashboard risers allows the
ponding depth to be adjusted, as described by Hunt et
al. (2007). Their Stormwater Wetland Design Update
(AG-588-12) provides design guidance for outlet struc-tures.
Further construction guidance for these systems is
provided by Burchell et al. (2010) in Stormwater Wet-land
Construction Practices (AG-588-13).
SUMMARY
Naturally-occurring wetlands are recognized as one of
the world’s most valuable ecosystems by virtue of the
free services they provide to society, including water
treatment; flood and greenhouse gas regulation; biodi-versity
maintenance; food and raw material production;
and recreational, educational, and aesthetic experiences.
Though currently designed for runoff treatment in North
Carolina and, to some degree, flood regulation, storm-water
wetlands may provide many of the other services
provided by naturally-occurring wetlands. Because they
are generally located in urban areas, stormwater wet-lands
have the potential to provide air and microclimate
regulation services. This potential in particular merits
further investigation by researchers and others involved
in stormwater mitigation. Carbon sequestration by
stormwater wetlands is another area for future explora-tion
with potential economic benefits for developers and
NC municipalities through the carbon market.
The types of ecosystem services provided by a
stormwater wetland will partly depend on its design.
Current design guidance, particularly including vari-ous
hydrozones and maintaining maximum water level
fluctuations to less than 6 inches while vegetation is
establishing, will encourage a more diverse community
of wetland vegetation. Much of the literature points to
this diversity as a driver for providing other ecosystem
services.
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RELATED FACT SHEETS
These fact sheets in the Urban Waterways series (AG-
588) published by the NC Cooperative Extension Ser-vice
at NC State University, Raleigh, are available on
the Stormwater Engineering website: http://www.bae.
ncsu.edu/stormwater/pubs.html
Hunt, W. F. and B.A. Doll. 2000. Designing Stormwater
Wetlands for Small Watersheds (AG-588-2).
Hunt, W. F., C. S. Apperson, and W. G. Lord. 2005.
Mosquito Control for Stormwater Facilities (AG-588-
4).
Hunt, W. F. and W. G. Lord. 2006. Stormwater Wetland
and Wet Pond Maintenance (AG-588-7).
Jones, M. P. and W. F. Hunt. 2007. Designing Urban
Stormwater BMPs for Trout Waters (AG-588-11).
Hunt, W. F., M. R. Burchell, J. D. Wright, and K. L.
Bass. 2007. Stormwater Wetland Design Update
(AG-588-12).
Burchell, M. R., W. F. Hunt, K. L. Bass, and J. D.
Wright. 2011. Stormwater Wetland Construction
Practices (AG-588-13).
Hathaway, J. M. and W.F. Hunt. 2008. Removal of
Pathogens in Stormwater (AG-588-16W).
NC STATE UNIVERSITY
Distributed in furtherance of the acts of Congress of May 8 and June 30, 1914. North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, veteran status or disability. In addition, the two Universities welcome all persons without regard to sexual orientation. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
Published by
NORTH CAROLINA COOPERATIVE EXTENSION
11-CALS-2352 AGW-588-22
1/11—BS/KEL
Prepared by
Trisha L. Moore, Graduate Teaching and Research Assistant
William F. Hunt III, Ph.D., PE, Associate Professor and Extension Specialist
Department of Biological and Agricultural Engineering